Drug delivery devices are well known and used widely in the treatment of patients. One form of such delivery devices is an implantable infusion pump which typically includes an expansible reservoir for containing a refillable supply of drug. Flow control and refill features are also provided on the pump. Pumps of this type are disclosed in U.S. Pat. Nos. 4,692,147 to Duggan and 5,445,616 to Kratoska et al, the subject matter of which is incorporated herein by reference. State-of-the-art pumps typically contain a propellant that exists in liquid and vapor phases to maintain a constant vapor pressure on the drug reservoir, which is typically an expansible metal bellows. The vapor pressure of the propellant remains substantially constant as the volume of the bellows and therefore the volume of the space between the bellows and pump housing changes.
Frequently, throughout the life and use of a drug delivery device, it is advantageous to periodically determine the volume of drug contained in the drug reservoir. For example, when an implantable pump is refilled, it is beneficial for a physician or technician to know the precise volume of drug remaining in the reservoir and to detect when the pump has been completely refilled. Likewise, periodic volume readings during pump operation permit a physician or possibly even a patient to determine when refilling should be scheduled. Such volume data can also be used to determine or possibly predict pump or infusion system malfunctions or leaks.
Prior art techniques for determining reservoir volume tend to rely on labor and time intensive methods. For example, during refill, the residual supply of drug in the pump may be determined by evacuating, to the extent possible, the residual supply of drug in the pump. Given the amount of effort and time involved in known techniques, it would be advantageous to provide a device for quickly, easily and accurately sensing the volume of drug contained in the reservoir at any desired time.